In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). Thus, there has been employed a chemical mechanical polishing apparatus for planarizing a surface of a semiconductor wafer. In the chemical mechanical polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing surface under a predetermined pressure by the substrate holding device. At this time, the polishing table and the substrate holding device are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface in the presence of slurry solution containing a polishing powder, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such polishing apparatus, if the relative pressing force applied between the semiconductor wafer, being polished, and the polishing surface of the polishing pad is not uniform over the entire surface of the semiconductor wafer, then the surface of the semiconductor wafer is polished insufficiently or excessively in different regions thereof depending on the pressing force applied thereto. It has been customary to uniformize the pressing force applied to the semiconductor wafer by providing a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplying the pressure chamber with a fluid such as air to press the semiconductor wafer under a fluid pressure through the elastic membrane, as seen in Japanese laid-open patent publication No. 2007-268654.
In this case, the polishing pad is so elastic that pressing forces applied to a peripheral portion of the semiconductor wafer being polished become non-uniform, and hence only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, the retainer ring for holding the peripheral edge of the semiconductor wafer is vertically movable with respect to the top ring body (or carrier head body) to press an annular portion of the polishing surface of the polishing pad that corresponds to the peripheral portion of the semiconductor wafer by the retainer ring.
In the conventional polishing apparatus such as an apparatus disclosed in the above-identified publication, a lateral force or horizontal force which works in a direction within the horizontal plane is applied to the retainer ring by a frictional force between the semiconductor wafer and the polishing surface of the polishing pad during polishing, and the lateral force (horizontal force) is received by a retainer ring guide provided at an outer circumferential side of the retainer ring.
1) Assuming the polishing apparatus has such structure, the polishing head is to have a fulcrum for receiving the lateral force (horizontal force) applied to the retainer ring by a frictional force between the semiconductor wafer and the polishing surface of the polishing pad in process of planarization of a substrate. In this apparatus, the fulcrum is to be positioned at the outer circumferential portion of the retainer ring. Because a contact area between the retainer ring and the retainer ring guide is limited (small area), in the case where the retainer ring is tilted and vertically moved to follow undulation of the polishing surface of the polishing pad, an unexpected large frictional force can be generated at sliding contact surfaces between an outer circumferential portion of the retainer ring and an inner circumferential portion of the retainer ring guide. Thus, the following capability of the retainer ring may become limited and insufficient in some cases, and there is a need for a polishing apparatus that has a capability of allowing a desired surface pressure of the retainer ring to be applied to the polishing surface of the polishing pad.
2) In the conventional polishing head in which a fulcrum of the retainer ring is positioned at the outer circumferential portion of the retainer ring and a rotary drive unit for transmitting a rotative force from the top ring (or carrier head) to the retainer ring is provided at the upper portion of the retainer ring, powder, or dried deposit generated after dry of a solution, may be generated at the fulcrum portion and the rotary drive unit due to sliding motion accompanying a frictional force. In the case where such powder falls down onto the polishing surface of a polishing table, defect such as scratch on the semiconductor wafer may be caused by the existence of the powder on the polishing surface in general. Thus, a member (boot) is an effective measure for preventing the powder from falling down. Irrespective of the merit, providing the member (boot) could have a demerit in terms of maintainability, because such boot is required to be reattached at the time of replacement of expendable articles, resulting in the possibility of tedious maintenance.
3) Because the retainer ring is thermally expanded during polishing, it is necessary to provide an adequate clearance between the retainer ring and the retainer ring guide. However, providing too wide clearance may cause an unexpected movement of the retainer ring, and abnormal noise or vibration tends to be generated at the time of collision between the retainer ring guide and the retainer ring caused by movement of the retainer ring in the clearance during polishing. Further, providing too wide clearance has another deficiency. If the retainer ring is off-centered with respect to the semiconductor wafer, variation in the polishing rate could be seen. For example, there could exist increases of the polishing rate at the outer circumferential portion of the semiconductor wafer in the circumferential direction of the semiconductor wafer.
4) Heat causes a thermal expansion of the retainer ring, and the heat is caused by friction between the retainer ring and the polishing surface. Thus, the retainer ring may spread outward toward the bottom due to a temperature difference and a linear expansion coefficient difference between the retainer ring and a drive ring to which the retainer ring is attached. If the semiconductor wafer is polished in this state, then the inner circumferential side of the retainer ring is to be worn faster than the outer circumferential side of the retainer ring, resulting in uneven wear of the retainer ring. Thus, the effect of the retainer ring for correcting the configuration of the pad surface of the polishing pad is not identical to between at the initial stage after replacement of the retainer ring, and at the stage of thereafter. Further, when a plurality of semiconductor wafers are sequentially processed, a temperature of the retainer ring gradually increases as the number of the processed semiconductor wafers increases during processing. In this case, thermal deformation quantity of the retainer ring gradually increases to cause the effect of the retainer ring to be changed between the processed semiconductor wafers. Furthermore, uneven wear of the retainer ring could occur and the effect of the retainer ring varies with time due to deformation of the retainer ring caused by a frictional force between the polishing surface and the semiconductor wafer.